CN107250407A - The hollow seamless steel pipe of spring - Google Patents

Abstract

A kind of hollow seamless steel pipe of spring, is in terms of quality %, to contain C：0.2~0.7%, Si：0.5~3%, Mn：0.1~2%, Cr：Higher than 0% and below 3%, Al：Higher than 0% and below 0.1%, P：Higher than 0% and below 0.02%, S：Higher than 0% and below 0.02% and N：Higher than 0% and below 0.02%, surplus is the hollow seamless steel pipe of iron and inevitable impurity, wherein, the wall thickness bias ratio calculated by following (1) formulas is less than 7.0%.Wall thickness bias ratio=(thickest-minimum wall thickness (MINI W.))/(average wall thickness)/2 × 100 (1).

Description

Hollow seamless steel pipe for spring

technical field

The present invention relates to a hollow seamless steel pipe for springs, and more particularly to a hollow seamless steel pipe for high-strength springs suitable for manufacturing hollow-shaped steel suspension springs used in automobiles and the like.

Background technique

In recent years, with the increasing demand for light weight and higher output power of automobiles aimed at reducing exhaust gas and improving fuel efficiency, suspension springs, valve springs, clutch springs, etc. used in suspension systems, engines, clutches, etc. , also working on high stress designs. Therefore, these springs tend to increase the load stress further in the direction of higher strength and smaller diameter. In order to cope with this tendency, there is a strong demand for a spring steel having higher performance also in terms of fatigue resistance and damping resistance.

In addition, in order to achieve weight reduction while maintaining fatigue resistance and ballistic resistance, hollow tubular steel materials and steel pipes without welded joints (hereinafter referred to as hollow seamless steel pipes) can be used as raw materials for springs instead of springs. The rod-shaped wire rod that has been used so far, that is, instead of the use of solid wire rod. Various techniques for producing such hollow seamless steel pipes have been proposed so far.

For example, Patent Document 1 proposes a technique of performing Mannesmann piercing, which is a representative piercing mill, on a raw material made of spring steel, followed by elongation rolling with a mandrel mill. , and then reheat at 820-940°C for 10-30 minutes, and then finish rolling. In addition, Patent Document 2 discloses a technique of producing a seamless steel pipe intermediate by subjecting a cylindrical billet to hot hydrostatic extrusion, heating the seamless steel pipe intermediate, and then treating the heated seamless steel pipe intermediate. The above-mentioned seamless steel pipe intermediate is subjected to at least one of periodic pipe rolling mill rolling and drawing processing, such as stretching to stretch it, and heating the stretched seamless steel pipe intermediate. In Patent Document 3, as in Patent Document 2, the hollow billet for extrusion is heated, then hot-extruded, and cold-worked to produce a seamless steel pipe. In addition, Patent Document 4 discloses a technique of manufacturing a seamless pipe by hot rolling a bar, piercing it with a gun drill, and performing cold rolling and drawing (cold working), thereby avoiding piercing and Heating during extrusion to reduce decarburization technology.

These prior arts intend to improve the fatigue characteristics by reducing decarburization and reducing defects, but a higher level of fatigue strength than the existing required level is currently required. Therefore, in these technologies proposed so far, the required fatigue strength cannot be satisfied, and the durability is insufficient. Especially in the higher stress region, the techniques proposed so far have limitations in improving the durability, and it is necessary to study other factors.

[Prior Art Literature]

【Patent Literature】

[Patent Document 1] Japanese Patent Application Laid-Open No. 1-247532

[Patent Document 2] Japanese Patent No. 4705456

[Patent Document 3] Japanese Patent Laid-Open No. 2012-111979

[Patent Document 4] Japanese Patent No. 5324311

Contents of the invention

The present invention was made under such circumstances, and an object of the present invention is to provide a hollow seamless steel pipe for a high-strength spring in which sufficient fatigue strength can be ensured for the formed spring.

The present invention which achieves the above-mentioned problems is characterized in that the variation in wall thickness of the steel pipe is reduced. That is, the hollow seamless steel pipe for springs of the present invention has the following points:

The hollow seamless steel pipe contains by mass %

C: 0.2 to 0.7%,

Si: 0.5~3%,

Mn: 0.1～2%,

Cr: more than 0% and less than 3%,

Al: more than 0% and less than 0.1%,

P: more than 0% and less than 0.02%,

S: Above 0% and below 0.02% and

N: more than 0% and less than 0.02%, the balance is iron and unavoidable impurities, of which,

The wall thickness deviation rate calculated from the following formula (1) is 7.0% or less.

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/(average wall thickness)/2×100 (1)

In the hollow seamless steel pipe for springs of the present invention, it is preferable that the maximum value of the deviation rate of wall thickness calculated by the following formula (2) is 7.0% or less, the depth of defects on the inner surface is 50 μm or less, and the inner surface is completely decarburized across its entire length. The layer depth is 100 μm or less.

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/{(maximum wall thickness + minimum wall thickness)/2}/2×100 (2)

The hollow seamless steel pipe for springs according to the present invention preferably further contains at least one of the following (a) to (f) in mass %, if necessary.

(a) B: more than 0% and less than 0.015%

(b) At least one selected from the group consisting of V: more than 0% and less than 1%, Ti: more than 0% and less than 0.3%, and Nb: more than 0% and less than 0.3%

(c) At least one selected from the group consisting of Ni: more than 0% and less than 3% and Cu: more than 0% and less than 3%

(d) Mo: more than 0% and less than 2%

(e) At least one selected from the group consisting of Ca: more than 0% and less than 0.005%, Mg: more than 0% and less than 0.005%, and REM: more than 0% and less than 0.02%

(f) One or more selected from the group consisting of Zr: more than 0% and less than 0.1%, Ta: more than 0% and less than 0.1%, and Hf: more than 0% and less than 0.1%

According to the present invention, since the wall thickness deviation rate, which is an indicator of the wall thickness deviation of the steel pipe, is highly reduced to 7.0% or less, it is possible to provide a high-strength hollow spring seamless steel pipe with high fatigue strength and excellent durability. The effect of the present invention can be remarkably exhibited particularly in a high-stress region.

Description of drawings

Fig. 1 is a graph showing the relationship between the ratio t/D of the wall thickness t of a steel pipe to the outer diameter D, and the variation rate of the inner surface stress due to the uneven wall thickness.

Fig. 2 is a graph showing the relationship between the ratio t/D of the wall thickness t of the steel pipe to the outer diameter D and the weight reduction rate.

FIG. 3 is a graph of the wall thickness deviation rate when the wall thickness tolerance is 0.1 mm for each wall thickness.

Fig. 4 is a view showing the shape of a test piece used in a torsional fatigue test in Examples described later.

FIG. 5 is a graph showing the relationship between the wall thickness deviation rate and the endurance number of torsional fatigue tests in Example 1 described later.

6 is a graph showing the relationship between the maximum value of the wall thickness deviation rate over the entire length of the steel pipe and the durability number of the torsional fatigue test in Example 2 described later.

detailed description

In high-strength hollow springs, there is a problem of improving the fatigue strength of the inner surface that cannot be shot-peened. So far, studies have been made on the suppression of decarburization and the reduction of defects on the inner surface. On the other hand, the inventors of the present invention intensively studied the influence of the wall thickness of the steel pipe as another influencing factor. As a result, it was found that the thickness deviation rate of the hollow steel pipe affects the fatigue strength.

In the conventional technologies such as the above-mentioned Patent Documents 1 to 4, the improvement of defects and decarburization is an important subject, but no consideration is given to the wall thickness deviation rate. However, as a result of studies conducted by the present inventors focusing on the wall thickness deviation rate, it has been found that the wall thickness deviation rate has a great influence on the fatigue characteristics, and in particular, by setting the wall thickness deviation rate to 7.0% or less, the durability of the seamless steel pipe can be improved. fatigue strength. The wall thickness deviation rate is preferably 5.0% or less, more preferably 3.0% or less. The smaller the wall thickness deviation rate, the better, but its lower limit is usually about 0.5%.

In addition, since the wall thickness is not uniform and the wall thickness variation rate varies across the entire length of the steel pipe, in order to obtain stable fatigue strength, it is considered preferable to suppress the wall thickness variation across the entire length. That is, in a preferred embodiment of the present invention, it has been revealed that the fatigue strength of the seamless steel pipe can be improved by setting the maximum value of the wall thickness deviation rate to 7.0% or less over the entire length of the steel pipe. The maximum value of the wall thickness deviation rate over the entire length of the steel pipe is more preferably 5.0% or less, still more preferably 3.0% or less. The wall thickness deviation ratio over the entire length of the steel pipe is preferably as small as possible, but the lower limit thereof is usually about 0.5%.

In the present invention, the wall thickness deviation rate is given by the following formula (1).

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/(average wall thickness)/2×100 (1)

Here, the maximum wall thickness and the minimum wall thickness mean the maximum value and minimum value of the wall thickness measured at multiple places in the same section, for example, 4 places measured every 90°, and the average wall thickness means the The average of the wall thicknesses measured at the multiple locations.

In addition, the wall thickness deviation rate over the entire length of the steel pipe is given by the following formula (2).

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/{(maximum wall thickness + minimum wall thickness)/2}/2×100 (2)

Here, the maximum wall thickness and the minimum wall thickness mean the maximum value and the minimum value of the wall thickness measured in one part, for example, by an ultrasonic probe or the like over the entire circumference of the steel pipe. The measurement of the wall thickness deviation rate using the formula (2) was carried out over the entire length of the steel pipe, and the maximum value of the obtained wall thickness deviation rate was taken as the "wall thickness deviation rate over the entire length of the steel pipe".

In addition, in the hollow seamless steel pipe for springs according to the present invention, the so-called "the ratio of wall thickness deviation calculated from the formula (1) is 7.0% or less", it is expected that the ratio of wall thickness deviation is approximately 7.0% substantially over the entire length of the steel pipe. %the following. Therefore, in a cross section extracted from an arbitrary portion such as a pipe end, for example, the wall thickness deviation rate calculated by the formula (1) is often 7.0% or less. Therefore, according to the results of a section, the wall thickness deviation rate calculated by formula (1) can be obtained.

In fact, in the above-mentioned Patent Documents 1 to 4, the wall thickness deviation rate cannot be said to be good. For example, in Patent Document 1, Mannesmann piercing is used to manufacture hollow steel pipes. Although Mannesmann piercing has high productivity, compared with other hollowing methods, there are constraints on materials and tools during hollowing, that is, during piercing. Weak, so it is easy to move, and it is difficult to obtain a good wall thickness deviation rate. In particular, steel materials for high-strength springs have high deformation resistance and are difficult to process with high precision. In addition, in Patent Documents 2 and 3, hot hydrostatic extrusion is performed on a machined hollow billet. Since machining is performed, the processing accuracy of the billet is high, and since it is uniformly processed by hydrostatic pressure, compared with Patent Document 1, the wall thickness deviation rate can be improved more easily. However, as shown in Examples described later, in the methods disclosed in Patent Documents 2 and 3, a sufficient thickness deviation rate cannot be obtained from the viewpoint of durability. Also, in Patent Document 4, gun drilling is employed as a hollowing method. In this method, the processing accuracy should be relatively good, but as shown in the examples described later, a sufficient wall thickness deviation rate cannot be obtained.

In addition, in a preferred embodiment of the present invention, in addition to controlling the above-mentioned wall thickness deviation rate, internal surface defects and total decarburization are adjusted across the total length of the tube, so that more stable fatigue characteristics can be obtained. The depth of internal surface defects across the total length of the tube is preferably 50 μm or less, and the total decarburization depth is preferably 100 μm or less.

In the hollow seamless steel pipe targeted by the present invention, the outer diameter D is about 8 to 22 mm, the wall thickness t is about 0.8 to 7.7 mm, and the ratio t/D of the wall thickness t to the outer diameter D is about 0.10 to 0.35.

Figure 1 shows the relationship between the ratio t/D of the wall thickness t to the outer diameter D and the variation rate of the inner surface stress caused by the uneven wall thickness under the wall thickness deviation rates of 3%, 7%, and 10%. Graph. The variation rate of the so-called inner surface stress is a value given by σ2/σ1 when the inner surface stress when there is no uneven wall thickness is σ1, and when the inner surface stress when there is uneven wall thickness is σ2. It can be seen from Fig. 1 that the internal surface stress variation rate when the wall thickness unevenness occurs becomes larger as t/D becomes higher. In addition, the variation rate of the inner surface stress, when t/D is low, even if the wall thickness deviation rate changes, the difference is small, but when the t/D is high, the influence of the wall thickness deviation rate on the variation rate of the inner surface stress become noticeable. As in the prior art, when the wall thickness deviation rate is higher than 7.0%, especially when t/D is 0.15 or more, the influence of the wall thickness deviation rate on the variation rate of the inner surface stress is large, that is, when t/D is 0.15 or more , the present invention is particularly effective.

In addition, FIG. 2 is a graph showing the relationship between t/D and weight reduction rate. It can be seen from Fig. 2 that the greater the t/D, the lower the weight reduction rate. In high-strength hollow springs, a weight reduction of 25% or more is required. Therefore, t/D is preferably 0.25 or less.

Fig. 3 is a graph showing the wall thickness tolerance for each wall thickness, that is, the wall thickness deviation rate when the difference between the maximum wall thickness and the minimum wall thickness is 0.1mm. It can be seen from Figure 3 that, for example, when the wall thickness is 0.5 mm, even if there is only a tolerance of 0.1 mm, it is equivalent to 10% if converted into a wall thickness deviation rate. In addition, in the prior art, due to the high wall thickness deviation rate Therefore, it is very difficult to improve the wall thickness deviation rate even when the wall thickness is thin.

The inventors of the present invention, as a manufacturing method for making the wall thickness deviation rate of the hollow seamless steel pipe to be 7.0% or less, particularly studied the method in which the following method (1) or (2) is used to manufacture A hollow blank tube, and then cold rolling, stretching, annealing, etc. are performed on the blank tube to obtain a hollow seamless steel tube.

(1) A method in which a hollow billet is obtained by mechanical processing from a rough billet, and the hollow billet is used for hot extrusion

(2) A method of hollowing out a steel bar by gun drilling after hot rolling it from a rough section

In the hot extrusion method of (1), the size of the hollow billet is changed, thereby changing the wall thickness deviation rate, so that the inner diameter of the hollow billet is 38mm, and the wall thickness deviation rate of the finally obtained seamless steel pipe can be realized. 7.0% or less of such rough tubes. On the other hand, in the aforementioned Patent Documents 2 and 3, the inner diameter of the hollow billet is 40 mm or 52 mm, and the wall thickness deviation rate of 7.0% or less cannot be achieved. In addition, in the method of using the gunhole drill of (2) above, the deviation rate of the wall thickness varies depending on the size of the bar steel and the size of the gunhole drill, and by performing the gunhole drilling process with a diameter of 20 mm on a steel bar with a diameter of 40 mm Therefore, it is possible to realize a blank pipe in which the wall thickness deviation ratio of the finally obtained seamless steel pipe is 7.0% or less. On the other hand, in the aforementioned Patent Document 4, a 12 mm-diameter gun-drilling process is performed on a 25-mm-diameter steel bar, and a wall thickness deviation rate of 7.0% or less cannot be achieved.

In addition, in the method of said (1), the heating temperature before hot extrusion should just be 1000-1100 degreeC, for example. In addition, in the method of (2) above, the heating temperature during hot rolling is, for example, about 950-1100°C, the minimum rolling temperature is 800-900°C, and the average cooling rate from 650-750°C after hot rolling It is about 1.5 to 5° C./second, and the average cooling rate to 500° C. or lower thereafter may be cooled at 0.3 to 1.0° C./second. In any one of the above (1) and (2) methods, the obtained blank tube is annealed for 5 to 30 minutes at, for example, 900 to 1000° C., cold rolled and stretched, and then heated at 600 to 1000° C. Annealing can be done left and right.

In order to more reliably reduce the wall thickness deviation rate to 7% or less over the total length, in the method (1) above, it was found that it is important to reduce the temperature difference in the longitudinal direction of the hollow billet during heating before extrusion, That is, thermal deviation is reduced. The heating time before hot extrusion is relatively short, and thermal deviation is likely to occur. Therefore, by performing soaking heating before heating, thermal variation is reduced, and the wall thickness variation rate over the entire length can be reduced. However, if the soaking heating temperature is too low or the soaking heating time is too short, the effect of reducing the wall thickness deviation rate will be lost, or the wall thickness deviation rate will increase. In addition, if the soaking temperature is too high or the soaking time is too long, decarburization will occur, and the total decarburization of the inner surface cannot reach 100 μm or less across the total length. Therefore, it is preferable that the soaking temperature is 900-950° C., and the soaking time is 300-2400 seconds. The soaking heating temperature is preferably 920°C or higher, preferably 940°C or lower. In addition, the soaking heating time is preferably 600 seconds or more, more preferably 1000 seconds or more, preferably 2000 seconds or less, more preferably 1500 seconds or less.

In addition, the heating temperature before extrusion is preferably 1100° C. or higher. If the heating temperature is lower than 1100° C., the frequency of occurrence of defects on the inner surface will increase, and it will be difficult to make the defects on the inner surface 50 μm or less across the entire length. This is considered to be because the higher the temperature, the higher the ductility during extrusion, and the less prone to defects. The upper limit of the heating temperature is not particularly limited, and may be, for example, about 1200°C.

In addition, the obtained blank tube can be annealed at 900-1000°C for 5-30 minutes, for example, and then cold-rolled and stretched, and then annealed at about 900-1000°C.

In the present invention, the thickness deviation rate of 7.0% or less can be achieved by the above-mentioned method, but the method of manufacturing the hollow seamless steel pipe of the present invention is not limited to the above-mentioned method.

Next, the chemical composition of the high-strength spring hollow seamless steel pipe of the present invention will be described. In addition, the chemical component composition shown in this specification is all the meaning of mass %.

C: 0.2 to 0.7%

C is an element necessary for securing strength, and the amount of C needs to be 0.2% or more. The amount of C is preferably 0.30% or more, more preferably 0.35% or more. However, if the amount of C is excessive, it will be difficult to ensure ductility. Therefore, the amount of C is made 0.7% or less. The amount of C is preferably 0.65% or less, more preferably 0.60% or less.

Si: 0.5-3%

Si is an effective element for improving the damping resistance required for a spring, and to obtain the damping resistance required for a spring of the strength level targeted by the present invention, the amount of Si needs to be 0.5% or more. The amount of Si is preferably 1.0% or more, more preferably 1.5% or more. However, Si is also an element that promotes decarburization, so if Si is contained in excess, the formation of a decarburization layer on the surface of the steel material is promoted. As a result, a peeling step for removing the decarburized layer is required, which is unfavorable in terms of production cost. Therefore, the amount of Si is set to be 3% or less. The amount of Si is preferably 2.5% or less, more preferably 2.2% or less.

Mn: 0.1-2%

Mn is used as a deoxidizing element, and forms MnS with S, which is a harmful element in steel materials, and is a beneficial element capable of making S harmless. In order to effectively exert such an effect, the amount of Mn needs to be 0.1% or more. The amount of Mn is preferably 0.15% or more, more preferably 0.20% or more. However, if the amount of Mn is excessive, segregation bands will be formed, and variations in the material will occur. Therefore, the amount of Mn is set to be 2% or less. The amount of Mn is preferably 1.5% or less, more preferably 1.0% or less.

Cr: more than 0% and less than 3%

Cr is an effective element for securing the strength and improving the corrosion resistance after tempering, and is particularly an important element for a suspension spring requiring a high level of corrosion resistance. Such an effect becomes greater as the amount of Cr increases. In order to effectively exert such an effect, it is preferable to contain Cr at 0.2% or more, more preferably at least 0.5%. However, if the amount of Cr is excessive, the supercooled structure is likely to be generated, and the cementite is concentrated to lower the plastic deformability, leading to deterioration of cold workability. In addition, when the amount of Cr is excessive, Cr carbides different from cementite are easily formed, and the balance between strength and ductility deteriorates. Therefore, the amount of Cr is set to be 3% or less. The amount of Cr is preferably 2.0% or less, more preferably 1.7% or less.

Al: more than 0% and less than 0.1%

Al is added mainly as a deoxidizing element. In addition, forming AlN with N makes solid solution N harmless and also contributes to microstructure refinement. In particular, in order to fix solid-solution N as AlN, it is preferable to contain Al more than twice the N content. The amount of Al is preferably 0.001% or more, more preferably 0.01% or more, and still more preferably 0.025% or more. However, Al, like Si, is also an element that promotes decarburization, and in steel containing a large amount of Si, it is necessary to suppress the amount of Al added. Therefore, the amount of Al is made 0.1% or less. The amount of Al is preferably 0.07% or less, more preferably 0.05% or less.

P: Above 0% and below 0.02%

P is a harmful element that degrades the toughness and ductility of steel materials, so it is important to reduce it as much as possible. Therefore, the amount of P is made 0.02% or less. The amount of P is preferably 0.010% or less, more preferably 0.008% or less. In addition, since P is an impurity unavoidably included in steel materials, it is difficult to make the amount 0% in industrial production, and it usually contains about 0.001%.

S: Above 0% and below 0.02%

Like P, S is a harmful element that degrades the toughness and ductility of steel materials, so it is important to reduce it as much as possible. Therefore, the amount of S is made 0.02% or less. The amount of S is preferably 0.010% or less, more preferably 0.008% or less. In addition, since S is an impurity unavoidably contained in steel materials, it is difficult for industrial production to make the amount 0%, and it usually contains about 0.001%.

N: Above 0% and below 0.02%

If Al, Ti, etc. exist, N will form nitrides with them and have the effect of refining the structure, but if N exists in a solid solution state, it will deteriorate the ductility and hydrogen embrittlement resistance of the steel material. Therefore, the amount of N is made 0.02% or less. The amount of N is preferably 0.010% or less, more preferably 0.005% or less. In addition, since N is an element that is inevitably contained in steel materials, it is difficult to make the amount 0% in industrial production, and it is usually contained at about 0.001%.

The basic components of the seamless steel pipe of the present invention are as described above, and the balance is substantially iron. However, unavoidable impurities mixed in due to the conditions of raw materials, materials, manufacturing equipment, etc. are naturally allowed to be contained in steel. In addition, in this specification, the balance of unavoidable impurities means unavoidable impurities other than the unavoidably included impurities of the above-mentioned respective elements except for the prescribed content.

In addition, in the present invention, any of the following elements may be contained as necessary.

B: Above 0% and below 0.015%

B has the effect of suppressing destruction from prior austenite grain boundaries after quenching and tempering of the steel material. In order to exhibit such an effect, the amount of B is preferably 0.001% or more, more preferably 0.0015% or more. However, if the amount of B is excessive, coarse carboborides are formed, which impairs the properties of the steel material, and also causes defects in the rolled material. Therefore, the amount of B is preferably 0.015% or less. The amount of B is more preferably 0.010% or less, still more preferably 0.005% or less.

One or more selected from the group consisting of V: more than 0% and less than 1%, Ti: more than 0% and less than 0.3%, and Nb: more than 0% and less than 0.3%

V, Ti, and Nb form carbides, nitrides, and carbonitrides (hereinafter referred to as carbon-nitrides) or sulfides with C, N, and S, and have the function of making these C, N, and S harmless. In addition, the above carbon/nitride also exerts the effect of making the structure finer. In addition, V, Ti, and Nb also have the effect of improving delayed fracture resistance. The amount of V is preferably 0.05% or more, more preferably 0.1% or more, and still more preferably 0.13% or more. Both the amount of Ti and the amount of Nb are preferably 0.03% or more, more preferably 0.04% or more, and still more preferably 0.05% or more.

However, if the amounts of V, Ti, and Nb are excessive, coarse carbon-nitrides are formed, and the toughness and ductility may be deteriorated. Therefore, it is preferable that the amount of V is 1% or less, the amount of Ti is 0.3% or less, and the amount of Nb is 0.3% or less. More preferably, the amount of V is 0.5% or less, the amount of Ti is 0.1% or less, and the amount of Nb is 0.1% or less. In addition, from the viewpoint of cost reduction, it is preferable that the amount of V is 0.3% or less, the amount of Ti is 0.05% or less, and the amount of Nb is 0.05% or less.

One or more selected from the group consisting of Ni: more than 0% and less than 3% and Cu: more than 0% and less than 3%

In consideration of cost reduction, the lower limit of Ni is not particularly set in order to control the addition, but it is preferable to contain 0.1% or more to suppress surface layer decarburization and improve corrosion resistance. However, if the amount of Ni is excessive, the properties of the steel material may deteriorate due to the generation of a supercooled structure and the presence of retained austenite after quenching in the rolled material. Therefore, when Ni is contained, the upper limit is preferably 3% or less. From the viewpoint of cost reduction, the amount of Ni is preferably 2.0% or less, more preferably 1.0% or less.

Cu, like Ni, is an element effective for suppressing decarburization of the surface layer or improving corrosion resistance. In order to effectively exert such an effect, Cu is preferably contained in an amount of 0.1% or more, more preferably 0.15% or more, and still more preferably 0.20% or more. However, if the amount of Cu is excessive, supercooled structures and cracks during hot working may occur. Therefore, when Cu is contained, the amount of Cu is preferably 3% or less. From the viewpoint of cost reduction, the amount of Cu is preferably 2.0% or less, more preferably 1.0% or less.

Mo: above 0% and below 2%

Mo is an effective element for securing strength after tempering and improving toughness. In order to exert such an effect, the amount of Mo is preferably 0.1% or more, more preferably 0.2% or more, and still more preferably 0.3% or more. However, when the amount of Mo is excessive, the toughness deteriorates. Therefore, the amount of Mo is preferably 2% or less. The amount of Mo is more preferably 1% or less, still more preferably 0.5% or less.

One or more selected from the group consisting of Ca: more than 0% and less than 0.005%, Mg: more than 0% and less than 0.005%, and REM: more than 0% and less than 0.02%

Ca, Mg, and REM (Rare Earth Metal, Rare Earth Element) all form sulfides, prevent the elongation of MnS, and have the effect of improving toughness, and can be added according to required characteristics. In order to effectively exhibit such an effect, both the amount of Ca and the amount of Mg are preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. The amount of REM is preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0012% or more. However, if the amounts of Ca, Mg, and REM are excessive, the toughness will be deteriorated on the contrary. Therefore, both the amount of Ca and the amount of Mg are preferably 0.005% or less, more preferably 0.004% or less, and still more preferably 0.003% or less. The amount of REM is preferably 0.02% or less, more preferably 0.01% or less, and still more preferably 0.005% or less. In the present invention, REM means 15 kinds of lanthanide elements from La to Ln, Sc and Y.

One or more selected from the group consisting of Zr: more than 0% and less than 0.1%, Ta: more than 0% and less than 0.1%, and Hf: more than 0% and less than 0.1%

Zr, Ta, and Hf combine with N to form nitrides, thereby suppressing the growth of austenite grain size during heating, making the final structure finer, and improving toughness. In order to effectively exert such an effect, the amount of Zr is preferably 0.01% or more, more preferably 0.03% or more, and still more preferably 0.05% or more. Both the amount of Ta and the amount of Hf are preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more. However, if the amounts of Zr, Ta, and Hf are excessive, the nitrides will be coarsened and the fatigue properties will be deteriorated, which is not preferable. Therefore, the amount of Zr is preferably 0.1% or less, more preferably 0.09% or less, still more preferably 0.05% or less, particularly preferably 0.025% or less. Both the amount of Ta and the amount of Hf are preferably 0.1% or less, more preferably 0.08% or less, still more preferably 0.05% or less, particularly preferably 0.025% or less.

【Example】

Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and within the range that can meet the above-mentioned and hereinafter-described purposes, of course, it can also be appropriately modified and implemented, and these are included in the technical scope of the present invention.

A molten steel having the chemical composition shown in Table 1 was smelted by a usual smelting method, cast and rolled to form a billet with a cross-sectional shape of 155 mm×155 mm. In addition, REM in Table 1 is added in the form of misch metal containing about 50% of La and about 25% of Ce.

(Table 1)

In the method of hot extrusion using a hollow billet, the above-mentioned billet is machined into a cylindrical hollow billet, and hot extruded to obtain a billet tube. Thereafter, cold rolling and stretching were performed to form a hollow seamless steel pipe with an outer diameter of 16 mm, an inner diameter of 8 mm, and a length of 3000 mm. The detailed production method is shown in A to D of Table 2.

In the method of making a steel bar by hot rolling and then hollowing it out by gun drilling, hot rolling is carried out from the above-mentioned blank under the conditions described in E and F of Table 2 to obtain a steel bar, which is processed by gun drilling. This was hollowed out to obtain a blank tube. Thereafter, cold rolling and stretching were performed to form a hollow seamless steel pipe with an outer diameter of 16 mm, an inner diameter of 8 mm, and a length of 3000 mm.

In addition, C in Table 2 is the production method disclosed in the above-mentioned Patent Document 3, D is the method disclosed in the above-mentioned Patent Document 2, and E is the method disclosed in the above-mentioned Patent Document 4.

(Table 2)

The hollow seamless steel pipes thus obtained were measured and evaluated by the following methods.

(1) Measurement of wall thickness deviation rate

The wall thickness of the pipe end of the hollow seamless steel pipe was measured at 4 points every 90° with a micrometer, and the wall thickness deviation rate was calculated from the following formula (1).

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/(average wall thickness)/2×100 (1)

(2) Evaluation of fatigue properties

The above-mentioned hollow seamless steel pipe was quenched and tempered under the following conditions.

Quenching conditions: After keeping at 925°C for 10 minutes, oil cooling

Tempering conditions: After keeping at 390°C for 40 minutes, water cooling

The quenched and tempered hollow seamless steel pipe was processed into a cylindrical test piece 1 shown in FIG. 4 . Fig.4 (a) is a front view, (b) is a side view which shows the end surface of a test piece. Using this cylindrical test piece 1, a torsional fatigue test was performed. The inner diameter of the test piece was about 8.0mm, the outer diameter of the constrained part 1a was 16mm, the outer diameter of the central part 1b was 12mm, and the load stress represented by the outer surface stress of the central part was 550±375MPa. The number of times until breaking was measured as the number of durability, and 10 ⁶ times of stopping tests were not broken.

The results are shown in Table 3 and Figure 5 . Fig. 5 is a graph showing the relationship between the wall thickness deviation rate and the endurance number of torsional fatigue tests in inventive examples and comparative examples of the present invention. (table 3)

Nos.1, 6-9, and 14-20 in Table 3 with a wall thickness deviation rate of 7.0% or less correspond to the circle marks in Fig. 5, and the number of endurance tests in the torsional fatigue test is 105 or more, showing good durability . In particular, Nos. 1, 6-9, 15-20 with a wall thickness deviation rate of 5.0% or less, and Nos. 1, 7-20 with a wall thickness deviation rate of 3.0% or less and a durability cycle of ⁵ ×105 or more. ⁹ , 15-17, 19, the number of endurance times is more than 106 times. On the other hand, Nos. 2 to 5 and Nos. 10 to 13 having a wall thickness deviation rate higher than 7.0% had a durable number of times less than 10 ⁵ as indicated by the X mark in FIG. 5 . Among them, Nos. 3 to 5 and 11 to 13 are examples manufactured under the manufacturing conditions C to E corresponding to the above-mentioned patent documents 2 to 4, and the result is that the wall thickness deviation rate exceeds 7.0%.

2. Embodiment 2

The molten steel with the chemical composition shown in Table 1 of Example 1 was smelted by a common smelting method, cast and billet-rolled to form a billet with a cross-sectional shape of 155 mm×155 mm. In addition, REM in Table 1 is added in the form of misch metal containing about 50% of La and about 25% of Ce.

Under the conditions of A to G shown in Table 4, a hollow blank pipe was obtained from the blank piece, and then cold rolled and stretched to produce a hollow seamless steel pipe with an outer diameter of 16 mm, an inner diameter of 8 mm, and a length of 3000 mm. Conditions A to F are a method in which a hollow billet is obtained by mechanical processing of the billet, and then hot-extruded to obtain a hollow billet tube. Condition G is a method of obtaining a bar steel by hot rolling the billet, and gun drilling is performed on it. Processing to obtain a hollow blank tube method. Condition E corresponds to the production method disclosed in Patent Document 3 above, F corresponds to the method disclosed in Patent Document 2 above, and G corresponds to the method disclosed in Patent Document 4 above.

(Table 4)

The hollow seamless steel pipes thus obtained were measured and evaluated by the following methods.

(1) Measurement of wall thickness deviation rate

For the hollow seamless steel pipe, the wall thickness was measured in the following manner.

(1-a) Wall thickness measurement at the end of the pipe

For the finally obtained hollow seamless steel pipe, the wall thickness of the end portion of the pipe was measured at 4 points every 90° with a micrometer, and the wall thickness deviation rate was calculated from the following formula (1).

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/(average wall thickness)/2×100 (1)

(1-b) Wall thickness measurement of total length

For the hollow seamless steel pipe, the wall thickness is measured over the entire circumference and length of the pipe by scanning an ultrasonic probe in contact with the outer surface of the steel pipe in the longitudinal direction of the steel pipe while rotating the steel pipe. According to the obtained wall thickness measurement results, the maximum wall thickness and minimum wall thickness when the probe scans the steel pipe for one cycle, the wall thickness deviation rate is calculated by the following formula (2). Across the total length, the wall thickness deviation rate is also measured to obtain the maximum wall thickness deviation rate.

At this time, the scanning speed of the ultrasonic sensor, the rotation speed of the tube, and the measurement pitch are adjusted so that the total length and the entire circumference can be inspected without omission. In addition, in order to ensure the quantitativeness, the correction of the ultrasonic measurement is performed before the inspection. Specifically, the end of the steel pipe is measured with a micrometer, and the ultrasonic measurement is corrected based on the result.

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/{(maximum wall thickness + minimum wall thickness)/2}/2×100(2)

(2) Measurement of internal surface defects

Similar to the wall thickness measurement of the overall length of (1-b) above, the depth of the defect on the inner surface of the entire circumference and the overall length of the pipe is measured with an ultrasonic probe. In addition, in order to ensure quantification, a standard tube with artificial defects of known size on the inner surface is used for off-line inspection and correction.

(3) Measurement of total decarburization of the inner surface

Decarburization was evaluated by cross-sectional observation. In order to evaluate variation in decarburization in the longitudinal direction, the steel pipe was divided into 10 equal parts, and 10 samples were extracted. After embedding the cut surface of the sample in resin for mirror grinding, it was etched with nital etching solution, observed with an optical microscope at a magnification of 200 times, and the maximum depth of the total decarburization layer depth on the inner surface of 10 samples was measured.

(4) Evaluation of fatigue properties

The above-mentioned hollow seamless steel pipe was quenched and tempered under the following conditions.

Quenching conditions: After holding at 925°C for 10 minutes, oil cooling

Tempering conditions: After keeping at 390°C for 40 minutes, water cooling

The quenched and tempered hollow seamless steel pipe was processed into a cylindrical test piece 1 shown in FIG. 4 . Fig.4 (a) is a front view, (b) is a side view which shows the end surface of a test piece. About this cylindrical test piece 1, the torsional fatigue test was performed using 10 pieces for each test No. each. The inner diameter of the test piece was about 8.0mm, the outer diameter of the restraint part 1a was 16mm, the outer diameter of the central part 1b was 12mm, and the load stress represented by the outer surface stress of the central part 1b was 550±375MPa. The number of times until the breakage is measured is regarded as the durability number, and the test is stopped if the number of times is not broken after 10 ⁶ times. Among the 10, the shortest endurance number is shown in Table 3 as the endurance number of each test No.

The measurement results of (1) to (4) are shown in Table 5 and FIG. 6 . 6 is a graph showing the relationship between the maximum value of the wall thickness deviation rate over the entire length of the hollow seamless steel pipe and the durability number of the torsional fatigue test in inventive examples and comparative examples of the present invention.

(table 5)

No. 1, 10, 12, 14, 23, and 25 in Table 3 spanning the total length of the steel pipe, the wall thickness deviation rate is less than 7.0%, the inner surface defect depth is less than 50 μm, and the inner surface total decarburization depth is less than 100 μm 27, 29, and 30 correspond to the circle marks in Fig. 6 , and the durability number in the torsional fatigue test is 10 ⁵ or more, showing good durability. In particular, the lower the wall thickness deviation rate, the more the number of endurance times tends to increase significantly. No. 10, 12, 14, 23, and 25 with the wall thickness deviation rate of 3.0% or less all reached 10 ⁶ or more endurance times.

On the other hand, Nos. 2, 4 to 8, 15, and 17 to 21 whose wall thickness deviation rate was higher than 7.0% corresponded to the X marks in FIG. 6 , and the number of endurance times decreased rapidly. However, Nos. 3, 9, 11, 13, 16, 22, 24 and 28, as shown by the triangular marks in FIG. 5 , the durability count is also low. In addition, in Nos. 6 to 8 and 19 to 21 manufactured under the manufacturing conditions E to G which are conventional techniques, all of them were results in which the wall thickness deviation rate was higher than 7.0%.

This application is accompanied by Japanese patent application No. 2015-001710 filed on January 7, 2015 and Japanese patent application No. 2015-001711 filed on January 7, 2015. Basic Priority Claims. Patent application No. 2015-001710 and patent application No. 2015-001711 are incorporated into this specification by reference.

The present invention includes the following aspects.

Method 1:

A hollow seamless steel pipe for springs, containing

C: 0.2 to 0.7%,

Si: 0.5~3%,

Mn: 0.1～2%,

Cr: more than 0% and less than 3%,

Al: more than 0% and less than 0.1%,

P: more than 0% and less than 0.02%,

S: Above 0% and below 0.02% and

N: A hollow seamless steel pipe of more than 0% and less than 0.02%, the balance being iron and unavoidable impurities, of which,

The wall thickness deviation rate calculated from the following formula (1) is 7.0% or less.

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/(average wall thickness)/2×100 (1) Method 2:

The hollow seamless steel pipe for springs according to aspect 1, wherein the maximum value of the wall thickness deviation rate calculated by the above formula (2) is 7.0% or less, and the depth of the defect on the inner surface is 50 μm or less across the entire length of the steel pipe, And the total depth of the decarburized layer on the inner surface is 100 μm or less.

Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/{(maximum wall thickness + minimum wall thickness)/2}/2×100 (2)

Method 3:

The hollow seamless steel pipe according to aspect 1 or 2 further contains B: more than 0% and not more than 0.015% in mass%.

Method 4:

The hollow seamless steel pipe according to any one of aspects 1 to 3, which further contains V: more than 0% and not more than 1%, Ti: more than 0% and not more than 0.3%, and Nb in mass %. : One or more species selected from the group consisting of more than 0% and less than 0.3%.

Way 5:

The hollow seamless steel pipe according to any one of aspects 1 to 4, further comprising Ni: more than 0% and not more than 3% and Cu: more than 0% and not more than 3% in mass %. Choose more than one of the group.

Method 6:

The hollow seamless steel pipe according to any one of aspects 1 to 5, which further contains Mo: more than 0% and not more than 2% by mass.

Way 7:

The hollow seamless steel pipe according to any one of aspects 1 to 6, further comprising, in mass %, Ca: more than 0% and not more than 0.005%, Mg: more than 0% and not more than 0.005%, and REM : One or more species selected from the group consisting of more than 0% and less than 0.02%.

Mode 8:

The hollow seamless steel pipe according to any one of aspects 1 to 7, which further contains Zr: more than 0% and not more than 0.1%, Ta: more than 0% and not more than 0.1%, and Hf in mass %. : One or more species selected from the group consisting of more than 0% and less than 0.1%.

【Industrial availability】

If the hollow seamless steel pipe of the present invention is used, a high-strength hollow spring with high fatigue strength and excellent durability can be produced, and the present invention can be applied, for example, to springs having a strength of 1100 MPa or higher, preferably 1200 MPa or higher, more preferably 1300 MPa or higher. Therefore, according to the present invention, hollowing of parts such as suspension springs, valve springs, and clutch springs can be promoted, and further weight reduction of vehicles such as automobiles can be achieved, so it is industrially useful.

【Description of symbols】

1 Cylindrical test piece

1a restraint department

1b central part

1c void

Claims (8)

1. A hollow seamless steel pipe for springs containing C: 0.2 to 0.7%, Si: 0.5~3%, Mn: 0.1～2%, Cr: more than 0% and less than 3%, Al: more than 0% and less than 0.1%, P: more than 0% and less than 0.02%, S: Above 0% and below 0.02% and N: A hollow seamless steel pipe of more than 0% and less than 0.02%, the balance being iron and unavoidable impurities, of which, The wall thickness deviation rate calculated by the following formula (1) is 7.0% or less, Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/(average wall thickness)/2×100 (1). 2. The hollow seamless steel pipe for springs according to claim 1, wherein the maximum value of the wall thickness deviation rate calculated by the following formula (2) is 7.0% or less across the entire length of the steel pipe, and the depth of the defect on the inner surface is 50 μm Below, and the total depth of decarburization layer on the inner surface is below 100μm, Wall thickness deviation rate = (maximum wall thickness - minimum wall thickness)/{(maximum wall thickness + minimum wall thickness)/2}/2×100 (2). 3. The hollow seamless steel pipe according to claim 1 or 2, further comprising B: more than 0% and not more than 0.015% by mass %. 4. The hollow seamless steel pipe according to claim 1 or 2, wherein V: more than 0% and less than 1%, Ti: more than 0% and less than 0.3%, and Nb are further contained in mass %. : One or more species selected from the group consisting of more than 0% and less than 0.3%. 5. The hollow seamless steel pipe according to claim 1 or 2, wherein, in mass %, it further contains Ni: more than 0% and not more than 3% and Cu: more than 0% and not more than 3% Choose more than one of the group. 6. The hollow seamless steel pipe according to claim 1 or 2, further comprising Mo: more than 0% and not more than 2% in mass %. 7. The hollow seamless steel pipe according to claim 1 or 2, wherein, in terms of mass %, Ca: more than 0% and not more than 0.005%, Mg: more than 0% and not more than 0.005%, and REM : One or more species selected from the group consisting of more than 0% and less than 0.02%. 8. The hollow seamless steel pipe according to claim 1 or 2, further comprising Zr: more than 0% and less than 0.1%, Ta: more than 0% and less than 0.1%, and Hf in terms of mass %. : One or more species selected from the group consisting of more than 0% and less than 0.1%.